A study of early holiday season emails by Experian Cheetah Mail revealed that the use of personalization in subject lines is up 21% over the same period last year. And with good reason - the emails with personalized subject lines show an improvement compared to non-personalized in open rate (4-6 percentage points), click rate (1-2 points), conversion rate (.3-.4 points) and revenue per email ($.12-$.22).